Polyolefin porous membranes are widely used as a separator in batteries, capacitors, and the like because the polyolefin porous membranes show high electrical insulation properties and ionic permeability. Particularly, in recent years, lithium ion secondary batteries of high power density and high capacity density are used as the power supply for portable devices as the functions of the portable devices are increased and the weight thereof is reduced. The polyolefin porous membrane is mainly used as a separator used for such lithium ion secondary batteries.
The lithium ion secondary battery has high power density and high capacity density. Because of an organic solvent used for an electrolytic solution, however, the electrolytic solution is decomposed by generation of heat accompanied by abnormal situations such as a short circuit and overcharge. This may lead to ignition in the worst case. In order to prevent such situations, some safety functions are incorporated into the lithium ion secondary battery, and one of them is shutdown function of the separator. The shutdown function is a function that micro pores of the separator are blocked by thermofusion or the like to suppress conduction of ions in the electrolytic solution and to stop progression of an electrochemical reaction when the battery generates the abnormal heat. Usually, it is supposed that the lower the shutdown temperature is, the safer the battery is. The proper shutdown temperature of polyethylene is one of the reasons that polyethylene is used as a component for the separator. A problem of a battery having high energy, however, is that the temperature within the battery continues increasing even if progression of the electrochemical reaction is stopped by shutdown; as a result, the separator is thermally shrunk and broken, causing a short circuit (short out) between two electrodes.
On the other hand, nonaqueous electrolyte batteries such as lithium ion secondary batteries have been increasingly applied to electric vehicles, hybrid electric vehicles, and the like, which need charge and discharge of a large amount of current in a short time. Such applications require not only safety but also advanced output properties. Namely, high safety and advanced output properties need to be satisfied at the same time.
Patent Literature 1 discloses a multilayer porous membrane including a porous membrane composed of a polyolefin resin as a main component and a porous layer laminated on at least one surface of the porous membrane wherein the porous layer contains not less than 50% and less than 100% inorganic filler as a mass fraction. The literature describes the techniques for satisfying high heat resistance for preventing short-circuit of electrodes and excellent shutdown function at the same time even if the amount of heat generated is large in abnormal heat generation.
Patent Literature 2 discloses a polyolefin microporous membrane having a fibril diameter of 40 to 100 nm, a micropore size of 50 to 100 nm, and a tortuosity of 1.4 to 1.8. The literature describes the techniques for attaining high ion conductivity and mechanical strength even if a polyolefin microporous membrane and a heat-resistant porous layer are formed into a composite membrane.